State of the art full duplex modems typically include echo cancellation apparatus in order to eliminate contamination of the received signal by echoes of the transmitted signal. Many such modems are also arranged to provide a local loop test to assess modem performance. During a test, the output of the modem's transmitter is intentionally allowed to reach the input of the modem's receiver via the hybrid circuit which normally connects the modem to the transmission line, and the connection to the transmission line is opened. In this configuration, a test signal sequence applied to the transmitter is cancelled by the echo canceller within the modem, rendering any test of proper receiver operation ineffective. Consequently, it has been common to disable the echo canceller during testing. However, this procedure does not then test operation of the echo canceller.